The present invention relates to a process for the roasting of finely-divided iron ores and concentrates containing non-ferrous metals such as zinc, lead, copper, gold and silver, especially pyrite and pyrrhotite concentrates and ores, preferably in a flash-smelting furnace, and for their chlorination in a separate stage in order to vaporize the non-ferrous metals as metal chloride compounds.
Several sulfidic non-ferrous metal ores frequently contain not only the metal mineral ores concerned but also iron sulfides, pyrite or pyrrhotite, which can be recovered separately in a more or less pure form by concentration techniques. The currently known methods for processing these iron sulfides are based on the classical dead roasting and the production of SO.sub.2 gas, or their thermal decomposition and the production of elemental sulfur. If the sulfidic concentrates are sufficiently pure, the obtained roasting residue is suitable for iron production. This proportion of iron ore is usually an important one in respect of the economy of the processes. If these iron sulfides are not obtained in a form sufficiently free from non-ferrous minerals, as is often the case, the obtained calcine is not as such suitable for use as iron ore but must be treated further for the removal of the metals, often valuable in themselves. Various chlorination methods have been studied and developed for this purpose.
Methods used on an industrial scale include only the so-called Kowa Seiko process, in which calcium chloride is mixed with the calcine and the mixture is pelletized and heated in a revolving tubular furnace by countercurrent heating to a temperature of approximately 1250.degree. C., whereby the non-ferrous metals sublimate as chlorides and are then recovered from the gases. The hematite pellets thus purified are a suitable raw material for an iron-smelting plant. This process is applicable only to a calcine which has been roasted to a state very low in sulfur, and the metal contents to be vaporized must not be very high (2.5% in total). Another disadvantage is the high heat requirement of the chlorination and sintering.
Other processes, at a pilot plant stage, include the Montedison, LDK and Outokumpu processes.
Montedison is a 3-stage process, in which the heating and final oxidation of the calcine are carried out in the first stage, the hematite is reduced to magnetite in the second stage, and the third stage comprises chlorination with an air-bearing chlorine gas at a temperature of approximately 950.degree. C., by oxidizing magnetite yielding the necessary heat. The reactors are fluidized-bed reactors and operate in a series. The gases from the chlorination are directed to a wash for the recovery of the chlorides. The finely-divided product obtained is pelletized and sintered separately. Oil must be used for the pre-heating, the reduction and the sintering of the pellets.
The LDK and Outokumpu processes are based on the chlorination of calcine with gaseous chlorine in a shaft furnace. The former utilizes pre-pelletized calcine and the latter finely-divided hot calcine directly. Both processes involve a problem in keeping the heating and cooling zones distinctly separate and on an industrial scale an even distribution of chlorine in the shaft furnace.
All these processes are characterized in that the roasting is performed at a temperature below the melting point of the product and that the chlorination is carried out in solid state with solid CaCl.sub.2 or chlorine gas, and that these must be used in considerable excess over the theoretical requirement. The calcine is either pelletized or sintered before the chlorination, as in the Kowa Seiko process, or after the chlorination and before being fed into the smelting plant, as in the Montedison process. External fuel must be used for this drying, heating and sintering. Only part of the heat of reaction contained in the concentrate is used in the process itself; it is used for maintaining the roasting temperature, and often heat is also stored in the vapor during the roasting and recovered.
Not only chlorination but also sulfatizing is used in processing certain types of iron sulfide concentrates. However, for example lead and noble metals cannot be recovered in the sulfatizing process, but they remain in the roasting residue. The calcium and barium present in the concentrate also sulfatize easily and, being insoluble, they bind sulfur in the calcine and thereby the grade of the iron ore is lowered.
The object of the present invention is to eliminate the above disadvantages and to provide a process for the treatment of finely-divided ores and concentrates to produce iron oxide suitable for iron production and non-ferrous chlorides from which valuable metals can be recovered by methods known per se, the process also being advantageous in terms of heat economy and environmental protection.